Australian media reported that Cartwright, a 21-year-old actor in the Australian television soap opera "Home and Away" was three months pregnant.
Hewitt's management confirmed the couple were expecting a child and Cartwright's publicist Amanda Fry also released a brief statement announcing the news.
"Their respective families are delighted with the wonderful news," Fry said.
Hewitt proposed to Cartwright after losing this year's Australian Open final to Russian Marat Safin following a whirlwind romance.
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Cartwright was also involved in a long-term relationship before splitting with actor Beau Brady late last year.
She was a regular fixture in the Hewitt family supporters box during the Australian Open but their sudden engagement caught everyone by surprise.
